(6?) 



end: 



Fig. 37. Ascidian tadpole of Clavelina. 



air. atriopore; c. mantle; cer.v. cerebral vesicle; e. eye-spot; end. endostyle; ep. epicardium; 

 h. heart; m. mouth; mu. muscle-cells; n.c. nerve-cord; not. notochord; ot. otocyst; St. stomach; 



sub.n. subneural gland. 



r. C. b. 



Fig. 37 A. T.S. ocellus of the free swimming tadpole stage of the sea squirt Ascidia nigra. 

 (Drawing from an electron micrograph.) 



The ocellus is situated in the posterior wall of the cerebral vesicle. It consists of three parts, a lens 

 cell, a pigment cell, and a retina. The lens cell usually contains three lens vesicles, which are spheres 

 of cytoplasm bounded by mitochondria. The pigment cell contains granules of melanin, which 

 protect the photoreceptor from stray light. The retinal cells have processes that penetrate the 

 pigment cell. They are similar to vertebral rods, composed of a pile of membranes, closely applied 



to the uaner edge of the lens cell. 

 a. p. attachment plaque, a membrane specialization thought to function as an anchor of the retinal 

 cell process to the pigment cell membrane ; b.m. basement membrane, the outer limit of the cerebral 

 vesicle; c.v. cavity of the cerebral vesicle; I.e. lens cell; l.v. lens vesicle; m. mitochondrion; p.c. 

 pigment cell; p.g. pigment granule; p.m. piled menbrane of photoreceptor part of the retinal cell; 

 r.c.b. retinal cell body; r.c.n. retinal cell nucleus; r.c.p. retinal cell process. 

 (From a preparation by N. Dilly.) 



